The present invention relates to a method and apparatus for reducing system pressure pulsation for positive displacement pumps, and more particularly to a method and apparatus for reducing pressure pulsation when pumping fluids having air or other gas highly entrained therein.
Screw pumps are positive displacement pumps which deliver a fixed volume of fluid for each rotation of the pump shaft. The configuration and operation of conventional screw pumps are well defined in the art, as briefly summarized below.
Screw pumps deliver a fixed volume of fluid by meshing at least two screws, which are bounded by a tight fitting housing, to create enclosed volumes which carry fluid through the pump. At the inlet of the pump, as the screws rotate, a volume is created and increases in size until at some point of rotation the volume is closed off from the suction end of the pump and becomes a fixed volume. This volume is transported to the discharge end of the pump by further rotation of the screws until the volume breaks into the discharge end of the pump. At this point, the volume is reduced in size by further rotation of the screws, forcing the fluid out of the pump.
These enclosed, fixed volumes are referred to as closures or sealed chambers which exist between the suction and discharge volumes. Screw pumps must have at least one closure, but may have multiple closures to attain higher pressures. When pumping high bulk modulus fluid, that is, fluid with little to no entrained gas, the small amount of leakage, due to manufacturing tolerances between the housing and screws, causes a staging of pressure from suction to discharge. This staging of pressure is fairly linearly distributed in the closures, with the differential between each closure equal to about the value obtained by dividing the discharge pressure by the number of closures plus 1.
As the screws rotate these closures move from suction to discharge, and, at a given point of rotation, the last closure opens into the discharge volume. Since the volume of the last closure is at a lower pressure than the discharge volume and since the fluid being pumped has a high bulk modulus, the combined volume undergoes a sudden decompression. There is a corresponding rebound resulting in positive pressure pulse. As most screw pumps have two thread starts on the screws, there are two closures opening to the discharge volume per revolution of the main screw. Therefore two pressure pulses are delivered per revolution and the primary pulsational frequency of the pump is two times pump speed.
All of existing prior art related to pulsation reduction in screw pumps is intended to reduce this primary pulsation level which causes fluid borne noise. Although this primary pulse is undesirable in certain low noise applications, the level of pressure pulse is not sufficient to cause damage to the pump or associated hydraulic system. Since the level of pulsation is inversely related to the bulk modulus of the fluid, screw pumps are typically utilized with high bulk modulus fluid so that the corresponding magnitude of the pressure pulse is small, typically 1 to 4% of discharge pressure. Additionally, although the associated noise is small, screw pumps are typically applied in applications where low fluid borne noise is a requirement, so there is a continuing effort to reduce this fundamental pulsation level.
U.S. Pat. Nos. 5,123,821 and 5,934,891 deal directly with this problem. U.S. Pat. No. 5,123,821 describes a method for tapering the screws in order to gradually precharge the last closure. It is well understood that precharging the last chamber will reduce primary pulsation pressure and many methods have been employed to accomplish this. U.S. Pat. No. 5,934,891 describes a method that is intended to control variation in the leakage flow within the pump. This is accomplished by utilizing two principles. The first is a controlling of the length of the screws by undercutting the screw diameter on either end of the pump. It is also well known that controlling the length of the screw set so that an exact integer number of closures are formed will minimize pulsation levels. In addition to this, the patent also refers to tapering the ends of the non-drive screws, which will have the same effect as U.S. Pat. No. 5,123,821.
U.S. Pat. Nos. 3,574,488 and 4,773,837 deal with a phenomenon known as cavitation. In this case minute fluid vapor bubbles are introduced at the suction end of the pump and are imploded as they are pressurized in the high pressure area of the pump. These gases exist in the fluid as defined by the laws of partial pressures. It should be noted the quantity of vapor is small and does not significantly change the bulk modulus of the fluid. Therefore, this problem can be solved by methods described above, namely precharging the last closure before it opens to the discharge volume. U.S. Pat. No. 3,574,488 accomplishes this by providing small drillings from the discharge port to the last closure. U.S. Pat. No. 4,773,837 does this by placing circumferential grooves on the driven screws.
Methods for reducing the primary pulsational pressure are generally only effective for a specific set of operating conditions. The magnitude of the reduction is affected by pump speed, fluid viscosity, pressure and standard pump clearances, which vary somewhat during manufacturing. Generally, testing must be performed to establish the feature parameters for a given set of operating conditions and this empirical data is not readily extrapolated to other operating conditions.
U.S. Pat Nos. 2,601,003; 4,233,005; 6,042,352; and 6,033,197 relate to gear pumps and address problems associated with trapped volumes and cavitation. These patents are not related to the problems addressed in this invention.
The prior art discussed above deals with high bulk modulus fluids and the reduction of the primary pulsation pressure which is in the order of 1-4% of discharge pressure. When pumping fluids with highly entrained gas, that is, greater than 2% by volume, the methods taught in the prior art are ineffective. Under these conditions the fluid no longer behaves like a high bulk modulus fluid, but instead becomes a low bulk modulus fluid.
The effect of highly entrained fluids on pressure pulsation is also dramatically different than with high bulk modulus fluids. The entrained air or gas is compressible and is capable or storing large amounts of energy. When the entrained fluid moves from the suction volume towards the discharge area, the normal linear pressure staging is disrupted. There is inadequate leakage flow in the pump to compress the gas sufficiently to permit this staging to occur. Subsequently, the fluid entering the discharge volume is only slightly above inlet pressure and, therefore, the normal pressure decompression is particularly intense.
When pumping these highly entrained fluids, pressure pulsations may reach as much as 400% peak to peak. In extreme cases, the pressure during the decompression interval of the pulse can drop below the vapor pressure of the fluid, and when this occurs the resulting rebound pressure can be extremely high. These high pressure pulses can cause catastrophic damage to the pump or associated hydraulic system.
What is needed is a method and apparatus for pumping fluids having a low bulk modulus. Also needed is for the device to significantly reduce pressure pulsation levels in the pump. Further needed, is for the reduction in pressure pulsation to be accomplished in a controlled predictable manner.
Applicant is aware of the following U.S. Patents concerning positive displacement pumps.
The present invention is a screw pump for pumping fluids highly entrained with gas. During operation, the inventive screw pump greatly reduces pressure pulsation in the screw pump by providing sufficient slip flow from the discharge end to the suction end. The slip flow compresses the entrained gas throughout the flow path of the pump so that the fluid pressure increases in a generally linear fashion as the fluid is conveyed from the suction end to the discharge end.
In the broadest sense, the screw pump includes a screw housing having a chamber with axially spaced inlet and outlet ends. A rotary threaded screw is positioned within the housing to convey fluid from the inlet end to the outlet end. A slip path extends between the ends to convey fluid from the outlet end to the inlet end.
Also in the broadest sense, the screw pump comprises a pump casing having a suction end, a discharge end, and a screw channel there-between. The screw channel includes a first bore and a second bore juxtaposed with the first bore. A drive screw and an idler screw are mateingly engaged and are respectively rotatably disposed within the first bore and the second bore. At least one of the clearances between the surfaces of the drive screw, idler screw and screw channel is larger than the remaining clearances and define a slip path extending from the discharge end to the suction end. Slip flow occurs within the slip path from the discharge end to the suction end. Optionally, the slip path can be disposed between the outer diameter of the drive screw and a wall of the first bore. In another option, the slip flow has a flow rate of about eight percent (8%), or more, of the pump""s through-flow rate.
Additionally in the broadest sense, a method of reducing pressure pulsation in a pump is provided. This method includes providing a pump having a casing with a suction end, a discharge end, and a screw channel there-between. The screw channel includes a first bore and a second bore juxtaposed with the first bore. A drive screw and an idler screw are mateingly engaged and are respectively rotatably disposed within the first bore and the second bore. At least one of the clearances between the surfaces of the drive screw, idler screw and screw channel is larger than the remaining clearances and define a slip path extending from the discharge end to the suction end. Fluid is provided at the suction end and then conveyed through the screw channel to the discharge end. Slip flow is conveyed within the slip path from the discharge end to the suction end. Optionally, the fluid pressure can be generally linearly increased as the fluid is conveyed from the suction end to the discharge end.
The principal object of the present invention is to provide a method and apparatus for reducing pressure pulsation in pumps.
A further object of this invention is to provide an apparatus for reducing pressure pulsation in low bulk modulus fluids.
Another object of the invention is to provide an apparatus which reduces vibrations in pumps.
A further object of this invention is to provide and apparatus which reduces fluid borne noise in pumps.
Another object of this invention is to provide an apparatus which has controlled slip flow from its discharge end to its suction end.
A further object of this invention is to provide an apparatus which uniformly and in a controlled fashion compresses entrained air as the fluid is carried through the pump.
Another object of the present invention is to provide an apparatus which provides linear compression of entrained air throughout the flow path in the pump.